The invention concerns a current transformer for alternating current particularly mains alternating current with direct current components consisting of at least one transformer core with a primary winding and at least one secondary winding to which a burden resistor is connected in parallel and terminates a secondary circuit with low resistance.
The power consumption of electrical instruments and apparatus in industrial and domestic use is measured by means of power meters. The oldest principle here utilized is that of the Ferraris wattmeter. The Ferraris wattmeter is based on measuring power through the rotation of a disk connected to a mechanical counter and driven by the current- or voltage-proportional fields of the respective field coils. In order to extend the capability of power meters, for instance for multiple tariff operation or remote control, use is made of electronic power meters in which current and voltage information is obtained by inductive current and voltage transformers. The output signals of these transformers are digitized, multiplied in-phase, integrated and stored. The result is an electrical dimension available for remote reading and other purposes.
On account of the frequently very high currents, that is currents in excess of 100A, the electronic power meters used for measuring power consumption in industrial applications operate indirectly. Special current transformers are connected in front of the current inputs so that only simple bipolar zero-symmetrical alternating currents have to be measured in the meter itself. The current transformers used for this purpose are designed with transformer cores made of highly permeable material. In order to obtain low errors in measurement over a small phase error, these transformers must be provided with very many secondary windings, that is typically more than 2500 secondary windings for 1 primary winding. These are unsuitable for use in domestic meters, which can also be installed in small industrial operations, because modern semiconductor circuits such as rectifier circuits or phase-angle circuits create current flows that are not zero-symmetrical and contain a direct current components This magnetically saturates the current transformer and thus falsifies the power reading.
Known current transformers for mapping such currents operate on the basis of open or mechanically applied air gaps and thus low-permeability magnetic circuits. Since, however, the noise immunity requirements of such current transformers must be very high in order to enable calibrated power measurement, these designs must be provided with costly shielding against external fields. This is demanding in terms of both material and assembly and hence is uneconomical for a wide range of domestic applications.
Another known possible concept is the use of current transformers with relatively impermeable transformer cores, that is transformer cores with permeability xcexc=2000. Such permeability avoids saturation with small direct current components. A difficulty with these types of current transformers is the balance between the highest non-falsified transmittable effective value of the bipolar zero-symmetrical sine current to be measured and the highest non-falsified transmittable amplitude of a unipolar half-wave rectified sine current. The international standard IEC 1036 applicable in this case provides a ratio for these two dimensions of 1:1.
Achieving this ratio requires the lowest possible permeability. This however causes a high phase error between primary and secondary currents where a practical number of windings is used. As this must be compensated for in the power meter, it requires an appropriate electronic circuit.
In hitherto known current transformer designs the range of compensation is limited to a phase error of 5xc2x0. In practice this causes the highest transmittable effective value to be necessarily vastly oversized. Ratios occur of 3-4:1. This leads to very poor use of materials and thus to very high production costs.
In addition this phase error must be maintained with very high linearity over the entire current range to be transmitted in order to keep the cost of compensation as low as possible.
The goal of the present invention, therefore, is to present a current transformer for alternating current with direct current components of the type mentioned at the outset that provides high controllability for both alternating current and direct current components.
In addition it should provide a highly linear transmittance ratio for precise current measurement over a wide current range.
Moreover it should show high immunity against external magnetic fields without additional shielding precautions so that it can be used economically with simple means, particularly with low mass transformer cores and low winding turn counts, especially for measuring the power consumption of domestic electrical instruments and apparatus.
The goal is achieved according to the invention by means of a current transformer for alternating current with direct current components consisting of at least one transformer core with a primary winding and at least one secondary winding to which a burden resistor is connected in parallel and terminates a secondary circuit with low resistance, specially characterized in that:
1. the transformer core comprises a closed ring core with no air gap produced from a strip (strip ring core) made of an amorphous ferromagnetic alloy;
2. the amorphous ferromagnetic alloy has a magnetostriction value |xcexs| less than 0.5 ppm and a permeability xcexc less than 1400; and
3. the alloy has a composition consisting essentially of the formula
Coa(Fe1-xMnx)bNicXdSieBfCg.
where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge and P, a-g are given in atomic % and whereby a, b, c, d, e, f, g and x satisfy the following conditions:
40xe2x89xa6axe2x89xa682; 2xe2x89xa6bxe2x89xa610; 0xe2x89xa6cxe2x89xa630; 0xe2x89xa6dxe2x89xa65; 0xe2x89xa6exe2x89xa615; 7xe2x89xa6fxe2x89xa626; 0xe2x89xa6gxe2x89xa63;
with 15xe2x89xa6d+e+f+gxe2x89xa630 and 0xe2x89xa6x less than 1.
These measures would produce a current transformer with excellent controllability for both alternating current and direct current components.
It would be further distinguished by a transmittance ratio with high linearity so as to ensure precise current measurement over a very wide current range. Moreover its design with no air gap would provide high immunity against external magnetic fields so that no additional shielding precautions would be necessary. The alloying system according to the invention would enable the achievement of very low mass transformer cores.
With a primary winding count of n1=1 current transformers can be produced with a secondary winding count of about 1500. Altogether according to the invention a current transformer can be produced at extremely low cost that tolerates direct current and is exceptionally suitable for the industrial and domestic applications mentioned at the outset
Particularly good current transformers can be produced through the use of amorphous ferromagnetic alloys having a magnetorestrictive value |xcexs| less than 0.1 ppm, and permeability xcexc less than 1200 where the alloy has a composition consisting essentially of the formula
Coa(Fe1-xMnx)bNicXdSieBfCg.
where X is at least one of the elements V, Nb, Ta, Cr, Mo, W, Ge and P, a-g are given in atomic % and whereby a, b, c, d, e, f, g and x satisfy the following conditions:
50xe2x89xa6axe2x89xa675; 3xe2x89xa6bxe2x89xa65; 20xe2x89xa6cxe2x89xa625; 0xe2x89xa6dxe2x89xa63; 2xe2x89xa6exe2x89xa612; 8xe2x89xa6fxe2x89xa620; 0xe2x89xa6gxe2x89xa63;
with 17xe2x89xa6d+e+f+gxe2x89xa625 and xxe2x89xa60.5.
The alloy systems mentioned above are characterized by linear, flat B-H loops up to a value of H=1 A/cm or greater. The alloy system according to the invention is practically free of magnetostriction. Magnetostriction is preferably suppressed by means of heat treatment whereby the actual saturation magnetostriction is obtained by fine adjustment of the iron and/or manganese content The saturation magnetostriction BS of 0.7 to 1.2 Tesla is enabled by fine adjustment of the nickel and glass-forming content. Glass-forming is here understood to mean X, silicon, boron and carbon.
Among the amorphous ferromagnetic cobalt-based alloy systems according to the invention, particularly suitable alloys have been shown to be those in which the parameter a+b+cxe2x89xa777 is adjusted to cxe2x89xa620. This enables saturation magnetostriction values BS of 0.85 Tesla or greater to be readily attained.
The permeability of less than 1400 arises from the physical relationship where permeability xcexc is inversely proportional to uniaxial anisotropy KU. The uniaxial anisotropy KU can be adjusted by means of heat treatment in a transverse magnetic field. The higher the content of cobalt, manganese, iron and nickel, the higher can the uniaxial anisotropy KU be adjusted. The nickel content here exerts an especially strong effect upon the uniaxial anisotropy KU.
To obtain low permeability an appropriate range of strip thickness for the strip ring core has been shown to be a thickness dxe2x89xa630 xcexcm, preferably dxe2x89xa626 xcexcm.
To obtain the best possible linear, flat B-H loop an [appropriate] strip thickness for the strip ring core has been shown to be a thickness d=xe2x89xa717 xcexcm. In alloys according to the invention this enables the surface-related component of the noise anisotropy to be very significantly reduced.
Typically the strip of the strip ring core has an electrically insulating layer on at least one surface. In another version the entire ring core has an electrically insulating layer. This enables the attainment of especially low permeability values as well as even greater improvement in B-H loop linearity. In selecting the electrically insulating medium, care should be taken that this adheres well to the surface of the strip while causing no reaction on the surface that could lead to degradation of magnetic properties.
Among alloys according to the invention oxides, acrylates, phosphates, silicates and chromates of the elements calcium, magnesium, aluminum, titanium, zirconium, hafnium and silicon have produced particularly effective and compatible electrically insulating media.
Among these, magnesium oxide is particularly effective and economical. It can be applied as a liquid, magnesium-containing precursor product on the surface of the strip. Then by means of a special heat treatment that does not affect the alloy it can be converted into a thick magnesium-containing layer with a thickness D between 25 nm and 400 nm. Actual heat treatment in a transverse magnetic field produces a well-adhering, chemically inert, electrically insulating layer of magnesium oxide.